It bears repeating: the world is complicated.
Oh, it looks simple enough: your coffee pours from pot to cup, round things roll but square things don’t, stuff that goes up will come back down (usually). But a little physics can peel back the skin of the world and give you a glimpse of fascinating stuff going on beneath the surface, driving the simplest of processes. It can make you into the kind of person who can stand in the shower shrouded in a too friendly plastic curtain for a good 15 minutes, pondering whether the thing is drawn inward thanks to the same phenomenon that helps keep planes in the air — Bernoulli’s Principle, as you know if you’re indeed one of those people.
Two new papers in the most recent Science take us to this deconstructionist place: they explore the knotty math behind bubbles and the secret lives of cirrus clouds, a pair of things that owe their existence to some very complex science even if you’ve never thought about them too hard.
Bubbles are more than just individual, poppable spheres floating through the air, of course — even if those are the most pristine expressions of the delicate form. They’re also part of the foam on your cappuccino, the froth of your shampoo, and in that state, they fall into a notoriously difficult class of problems called multiphase multiphysics. Each bubble in a mound of foam is simply a tiny bit of fluid stretched around a pocket of air — air that presses out with equal pressure in all directions and thus gives the bubble its shape. Collectively, however, the bubbles operate on different scales governed by different physical rules.
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At the smallest scale, the bubbles may look still, but the fluid in each one is in a constant state of motion, gradually draining toward the places the bubbles meet. When that flow causes the skin of an individual bubble to grow too thin, it pops. In response to that, at a larger level, the whole cluster shifts and realigns until the arrangement of bubbles are stable again. Lather, rinse, repeat — foams go through this process over and over again until they melt away.
What the researchers behind the Science paper, Robert Saye and James Sethian of the University of California, Berkeley, have done is come up with a way to model foam’s decay, using a set of interlocking equations that describe each stage of the process in terms of what happened in the previous stage. With this, they generated this beautiful little movie of a cluster of bubbles draining, popping and adjusting. And in a bit of visual pyrotechnics, because they understood the physics of the bubbles’ surfaces so well, they were able to overlay a realistic sunset on each one, as if it were being reflected.
The other study takes us to a more rarefied place than the bathtub, miles up into the atmosphere, where, over the course of nine years, a team of investigators has sent research planes to collect samples of wispy cirrus clouds. These clouds can form much higher than passenger aircraft fly — in some parts of the world 18 km above ground — and their inaccessibility has kept us from knowing as much about them as we do about most other clouds. We know that the water droplets and ice crystals that make up clouds form around tiny particles in the atmosphere — for low-hanging clouds, the water often sticks to little gobs of sulfate or carbon, which can be emitted by volcanoes, wildfires and human industrial activities. To see what cirrus clouds were forming around, the team captured their component ice crystals in a cuplike catcher mounted on the research planes’ noses, melted them and examined their findings in the lab. They discovered mainly mineral dust and flecks of metal.
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That these high clouds are made of such things is significant, because mineral dust and metal can have both natural and human-mediated origins. Mineral dust gusts up off the Sahara Desert, for example, but it also drifts up from construction sites and other areas of disturbed land. High-altitude metal can also be produced naturally, but is likelier to be the exhaust of smelting operations that have risen into the atmosphere. Their concentrations in the air have been profoundly changed by human presence, says Daniel Cziczo, an atmospheric scientist at MIT and an author of the paper. “Some estimates say there’s maybe 50% more mineral dust now than there was before industrial times,” he says. “This means that there can be a human effect on these clouds.”
Clouds are among the biggest unknowns in understanding how climate works. The physical rules for forming ice crystals around particles of dust and metal differ from those that apply when the particles are made of, say, sea salt or carbon. The fact that dust and metal seeding takes place unexpectedly high in the sky tells meteorologists new things about the precise humidity and temperature at which cirrus clouds form. That knowledge will be useful to modelers looking for a more detailed understanding of the intricate, interconnected systems of our atmosphere.
It’s a good thing, of course, that fathoming the physics of cirrus clouds has such an important real-world application — just as fathoming the physics of bubbles can help chemists develop better foams for insulation, fire extinguishers or other industrial purposes. But that’s for the lab rats. For everyone else, it’s all about the coolness — which is a lot less practical, but a whole lot more fun.
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